This invention generally relates to thermoplastic material melting and dispensing technology and, more specifically, to devices for melting and dispensing so-called hot melt adhesive materials.
Thermoplastic adhesive materials which are heated and liquified just prior to their application on a substrate are referred to as "hot melt" adhesives. Hot melt adhesives are available in bulk in such forms as chicklets, pellets, slats and bricks, and are also available in the form of a slug contained in a pail or drum. A drum of bulk adhesive material may be as large as a conventional 55 gallon drum for large scale applications. The present invention is directed at solving various problems encountered with melting adhesive which is initially contained as a slug in such a pail or drum.
Typically, hopper-type dispensers have been used to both remove the slug of adhesive from the drum or container and to melt the slug of adhesive as it contacts a heated melting grid at the bottom of the hopper. A heated reservoir is disposed beneath the melting grid and receives the melted or liquified hot melt adhesive as it passes through the grid. The reservoir maintains the adhesive in a heated liquid state suitable for the application. An outlet of the reservoir typically leads to a pump for pumping the hot melt adhesive to a dispensing apparatus, appropriate for the application.
The melting grid of hopper-type dispenser units must be of sufficient strength to receive the heavy load of a slug of adhesive from a 55 gallon drum. Such slugs of adhesive may weigh up to several hundred pounds. On the other hand, the grid must be formed with intersecting grid members narrow enough to provide openings therebetween which allow a high throughput volume of liquified hot melt adhesive. If the grid members are made too narrow, however, the weight of the slug of adhesive will cause them to crack, bend or break and reduce the life of the grid. Therefore, a trade-off has existed between providing high throughput volume of liquified hot melt adhesive and providing a strong, sturdy melting grid with a long useful life.
Another characteristic of hopper-type dispensing units in need of attention and improvement relates to the difficulties encountered when replacing one drum with a new drum of adhesive after the adhesive slug in the first drum has moved downwardly into the hopper. This changeover procedure must occur rapidly enough that the hopper reservoir does not become depleted of adhesive before the next slug of adhesive is in place and being melted by the melting grid. Presently, the adhesive which is left in the hopper unit when the first drum is removed and a second drum is being inserted may not be of sufficient volume to enable the next drum to be safely and deliberately inserted without the hopper reservoir becoming depleted of hot melt adhesive. When the reservoir empties of hot melt adhesive before the next slug is being melted by the melting grid, the adhesive residue within the hopper unit may begin to degrade, burn or char.
In view of the above, it would be desirable to provide a hopper-type dispensing unit which reduces the stress and load placed on a melting grid, provides a high throughput volume of liquified hot melt adhesive, and allows adequate time for replacing an empty drum with a new drum of adhesive without becoming depleted of adhesive during the replacement process.